The present invention relates to a device for use in a cleaning installation for removing soot or similar inside deposits in a flow channel in a processing system such as a boiler, heat exchanger, flue gas filter or the like, by intermittently blasting a fluid or gaseous medium into the flow channel in the processing system, said device comprising a flow passage between associated valve means and the processing system. The invention also relates to a cleaning installation of such kind.
Boiler installation sediments of soot or the like often occur on the inside of heat surfaces and on tubes that are in contact with flue gases. This seriously reduces the thermal conductivity and thereby also the efficiency of the boiler if not removed.
For removal of these deposits, methods are known, such as mechanical cleaning or blast cleaning using stream, water or air as cleaning means for the removal of soot. Some of these cleaning methods require shut-down of the boiler installation whilst other methods allow for the boiler to remain in operation at a reduced level of efficiency. By a few methods such as blast cleaning with air, the cleaning process can be performed during ordinary operation of the boiler. An example of another of such systems is known from EP-A-0 865 023 where a sound generator is used for loosening the sediments.
The cleaning equipment is often subjected to a heat and corrosive impact from the gases inside the boiler installation. This results in a limited durability of the cleaning equipment due to the aggressive environment in the flue gases. For this reason, the cleaning equipment is often mounted on the outside of the gas chamber of the boiler.
Equipment for blast cleaning for the removal of soot by pressurized air, is attractive in use since the equipment only requires a small amount of space outside the boiler and none at all inside the boiler.
The cleaning equipment typically comprises a pressure vessel from which pressurized air is fed to a number of diaphragm valves. The valves are controlled by control means. The control means activates the valves in a pre-programmed sequence and the pipes in the boiler governed by the associated valve are blasted with the pressurized air.
The air is blasted for 0,1-1 second at an interval of approx. 2-10 minutes. This cleaning cycle, i.e. the duration and the frequency, can be adjusted according to requirements, such as the type of fuel, the amount of fouling and the boiler output. The pressure vessel is usually operated at about 7-8 bars. However, this can also be adjusted in accordance with the actual needs of the boiler installation.
However, permanent mounting of the blasting tubes in the boiler wall causes a deterioration of the components in the soot removal equipment exposed to the gases inside the boiler and in particular to valve and membrane components.
There are many different types of boilers in power plants. Particularly municipal waste burning plants, bio-mass fuelled plants and waste heat recovery installations generate corrosive flue gases during the burning process.
The components are in particular subject to corrosion due to the hot, aggressive flue gases during the off-cycles, i.e. when no air is blasted through the valve assembly, the associated tube passage and into the boiler. When there is no forced air flow in the valve assembly, flue gas will escape from the main gas stream in the boiler and fill the passage space. This causes mechanical wear and corrosion of the exposed components of the cleaning equipment which, in turn, reduces the cleaning efficiency and again causes a drop in the efficiency of the boiler installation.
Therefore, the object of the invention is to provide a device that reduces corrosion of the exposed components of the cleaning equipment in a soot cleaning installation.
This object is achieved by a device and a cleaning installation of the initially mentioned kind wherein the flow passage is provided with an inlet allowing for a continuous flow of protective gas around the flow passage into the flow channel in the processing system.
By the present invention, corrosion is reduced since a cushion of air is generated at the end of the flow passage during the off-cycles of the cleaning operation, as air is constantly drawn or forced in through the inlet. This means that the components are covered by this cushion and protected against the corrosive gases in the heat exchanger or the like. Hereby, the valve means are protected from corrosion and tests have shown that durability of the exposed components facing the inside of the boiler or the like has been improved significantly. This also means that the components, such as the valve housing, may be produced in a cheap material such as cast iron without compromising the durability of the valve means.
In the preferred embodiment of the invention, the flow passage comprises an annular space between an inner tubular and an outer tubular member, where the outer tubular member is provided with an air inlet generating a continuous air flow around the flow passage into the processing system. Hereby, an even distribution of the air flow is ensured resulting in a particularly well-covering cushion of air.
In a first embodiment of the invention, the air is simply drawn in around the flow passage and into the inside end region of the flow passage utilizing the Venturi effect due to the speed of the gas flow in the boiler, heat exchanger or the like. However, in an alternative embodiment, the air inlet is provided with an injector for the supply of air into the flow passage. This means that air can be forced into the air inlet for the creation of the protective air cushion at the inside end section, even if the sub-pressure created by the flow rate of the gases or the like is insufficient for the creation of an air cushion.
In another embodiment, the device is provided with a plurality of air inlets. Hereby, the amount of air can be enhanced and the shape of the air cushion can be adjusted according to the radial position of the air inlets around the flow passage.
In a preferred embodiment of the invention, the air inlet is provided with replaceable flow restriction means. In a particular embodiment, the replaceable flow restriction means may consist of a cover plate provided with one or more apertures through which air is allowed to flow into the annular spacing. Hereby, the continuous inlet air flow can be restricted so that the amount of incoming air is not influencing the gases inside the boiler.
In another aspect of the invention, a valve assembly is provided for use in a cleaning installation for removing soot or similar inside deposits in a flow channel in a processing system such as a boiler, heat exchanger, flue gas filter or the like, by intermittently blasting a fluid or gaseous medium into the processing system, said valve assembly comprising a diaphragm valve including a first and second chamber, said first chamber receiving pressurized air from an air supply, and said second chamber being provided with valve control means and an outlet for release of air in the chamber when activating the diaphragm valve, and a valve outlet connected with a flow passage device according to the first aspect, wherein flow communication means is provided between the outlet of the second chamber and the inlet of the flow passage of the device.
When the diaphragm valve releases pressurized air which is typically up to 8 bars or perhaps more, the diaphragm retention air is released from the second chamber through the outlet and into the atmosphere. This creates a loud explosion-like noise, that can be more than 100 dBA. However, by a valve assembly according to the invention, a silencer is provided whereby noise emission is reduced considerably.
In the preferred embodiment of a valve assembly according to the invention, the flow communication means includes a tubular pipe. Hereby, the noise reduction may be obtained in a simple and reliable manner, just as a tubular pipe is relatively easy to incorporate in existing valve arrangements.
In another embodiment of the valve assembly, the flow communication means includes an additional supply of external air to the inlet of the flow passage device and this additional air supply preferably consists of cool air. Hereby, a constant air flow to the flow passage may be ensured, just as heat generated during the retention pressure release may be removed by an additional supply of cool air.